The present invention relates to a modified Karl-Fischer reagent useful in the determination of water. It contains a salt, sulfur dioxide and iodine. It also relates to a procedure for the determination of water using this reagent.
A number of proposals are known from the literature for replacing pyridine in the Karl-Fischer reagent by other substances. In Anal. Chim. Acta 94, 395 (1977), sodium acetate is used as a substitute for pyridine. However, this substitution involves certain disadvantages. The acetate is formed, for example, with the alcohol used as solvent. Water is liberated. This obviously interferes with a method of determination of water. For this reason, the solutions are not stable and their blank value increases continuously.
In British Pat. No. 728,947, alcoholates, phenolates and metal salts of weak organic acids are mentioned in addition to acetates as substitutes for pyridine. A re-check of the substances mentioned in the patent specification showed that these are not suitable as substitutes for pyridine, in some cases because of inadequate solubility and in some cases because of insufficient stability of the corresponding ready to use solutions. Furthermore, it is known that on replacing pyridine by amines, stable end products cannot be obtained upon titration (Anal. Chem. 28, 1166 (1956)). DOS 30 48 237 (U.S. Ser. No. 333,100 of Dec. 21, 1981) discloses a modified Karl-Fischer reagent wherein pyridine is replaced by ammonia or an alkali or alkaline earth metal benzoate.
In order to avoid these disadvantages, attempts have very recently been made to replace pyridine by aliphatic amines at a particular molar ratio to sulfur dioxide (German Offenlegungsschrift 3,010,436 corresponding to U.S. application Ser. No. 245,405 of Mar. 19, 1981) or by heterocyclic compounds (European Pat. No. 35,066). However, these pyridine substitutes again do not provide the desired results, since the stability of the end point varies with the amount of water to be titrated.